Densely-packed films of lanthanide oxide nanoparticles via electrophoretic deposition

ABSTRACT

A method of forming a film of lanthanide oxide nanoparticles. In one embodiment of the present invention, the method includes the steps of: (a) providing a first substrate with a conducting surface and a second substrate that is positioned apart from the first substrate, (b) applying a voltage between the first substrate and the second substrate, (c) immersing the first substrate and the second substrate in a solution that comprises a plurality of lanthanide oxide nanoparticles suspended in a non-polar solvent or apolar solvent for a first duration of time effective to form a film of lanthanide oxide nanoparticles on the conducting surface of the first substrate, and (d) after the immersing step, removing the first substrate from the solution and exposing the first substrate to air while maintaining the applied voltage for a second duration of time to dry the film of lanthanide oxide nanoparticles formed on the conducting surface of the first substrate.

CROSS-REFERENCE TO RELATED PATENT APPLICATION

This application is a continuation-in-part of U.S. patent applicationSer. No. 12/587,401, filed on Oct. 5, 2009, entitled “ElectrophoreticFabricated Feedstranding All-Nanoparticle Thin Film Materials” by SaadHasan and James Dickerson, which is incorporated herein by reference inits entirety.

Some references, which may include patents, patent applications andvarious publications, are cited in a reference list and discussed in thedescription of this invention. The citation and/or discussion of suchreferences is provided merely to clarify the description of the presentinvention and is not an admission that any such reference is “prior art”to the invention described herein. All references listed, cited and/ordiscussed in this specification are incorporated herein by reference intheir entireties and to the same extent as if each reference wasindividually incorporated by reference. In terms of notation,hereinafter, bracketed “n” represents the nth reference cited in thereference list. For example, [71] represents the 71st reference cited inthe reference list, namely, [71] S. V. Mahajan and J. H. Dickerson,Nanotechnology 18, 325605 (2007).

FIELD OF THE INVENTION

The present invention relates generally to films of nanoparticles, inparticular, to films of lanthanide oxide nanoparticles and methods offorming same.

BACKGROUND

Lanthanide oxides such as europium oxide (Eu₂O₃) and gadolinium oxide(Gd₂O₃) are known for their light emitting and high-K dielectricproperties, respectively [76, 58, 77]. The Eu³⁺-doped Gd₂O₃, inmicrocrystalline form, has been employed in video displays and tri-colorfluorescent lamps as a red phosphor [78]. Recently, nanocrystalline formof Eu³⁺-doped sesquioxides has gained research interest due to theirpotential use in luminescent biological tags, efficient light emittingdevices, and high-resolution displays. Gd₂O₃ has received researchattention because of its high-κ dielectric properties. Gd₂O₃ has beenproposed as silicon dioxide replacement for gate oxide in ultra-smallcomplementary metal-oxide-semiconductor (CMOS) devices [77]. Mostapplications of luminescent and dielectric materials require theirimplementation in thin-film form. The Eu₂O₃ and Gd₂O₃ nanocrystals, madevia colloidal techniques, need to be assembled into thin-film form tostudy their optical and dielectric properties.

Therefore, a heretofore unaddressed need exists in the art to addressthe aforementioned deficiencies and inadequacies.

SUMMARY OF THE INVENTION

In one aspect, the present invention relates to a method of forming afilm of lanthanide oxide nanoparticles. In one embodiment, the methodincludes the steps of: (a) providing a first substrate with a conductingsurface and a second substrate that is positioned apart from the firstsubstrate, (b) applying a voltage between the first substrate and thesecond substrate, (c) immersing the first substrate and the secondsubstrate in a solution that comprises a plurality of lanthanide oxidenanoparticles suspended in a non-polar solvent or apolar solvent for afirst duration of time effective to form a film of lanthanide oxidenanoparticles on the conducting surface of the first substrate, and (d)after the immersing step, removing the first substrate from the solutionand exposing the first substrate to air while maintaining the appliedvoltage for a second duration of time to dry the film of lanthanideoxide nanoparticles formed on the conducting surface of the firstsubstrate.

In one embodiment, the first substrate is gold-coated glass, gold-coatedsilicon, stainless steel (316L), indium-tin-oxide (ITO)-coated glass, ordoped silicon.

In one embodiment, the applied voltage, V, is in the range of 0volts<V≦1000 volts.

In one embodiment, the non-polar solvent or apolar solvent includes atleast one of hexane, octane and mixtures thereof, and each of the firstduration of time, T1 and the second duration of time voltage, T2, is inthe range of 0 minutes<T1, T2≦30 minutes.

In one embodiment, the first duration of time, T1 and the secondduration of time voltage, T2, can be same or different.

In one embodiment, the solution has a concentration ranging from about1×10¹⁴ nanoparticles per cubic centimeter to about 10×10¹⁵ nanoparticlesper cubic centimeter.

In one embodiment, the film of lanthanide oxide nanoparticles formed onthe conducting surface of the first substrate has a thickness rangingfrom about 50 to about 500 nm.

In one embodiment, the film of lanthanide oxide nanoparticles formed onthe conducting surface of the first substrate has randomly close-packedlanthanide oxide nanoparticles with a packing density of about 66%.

In one embodiment, the lanthanide oxide nanoparticles are europium oxide(Eu₂O₃) nanoparticles or gadolinium oxide (Gd₂O₃) nanoparticles.

In yet another embodiment, the lanthanide oxide nanoparticles have acore diameter ranging from about 2 to about 3 nm.

In a further embodiment, the lanthanide oxide nanoparticles aresurface-passivated with oleic acid.

In another aspect, the present invention provides an article ofmanufacture having a film of the lanthanide oxide nanoparticles made bythe method set forth immediately above.

In yet another aspect, the present invention relates to ametal-oxide-semiconductor (MOS) capacitor. In one embodiment, the MOScapacitor has: (a) a silicon substrate having a first surface, (b) afilm of lanthanide oxide nanoparticles formed on the first surface ofthe silicon substrate using the method set forth immediately above, and(c) an aluminum film formed on the film of lanthanide oxidenanoparticles, wherein the film of lanthanide oxide nanoparticlescomprises randomly close-packed lanthanide oxide nanoparticles with apacking density of about 66%.

In one embodiment, the silicon substrate has p-type silicon.

In one embodiment, the first surface of the silicon substrate is ap−(100) surface of silicon.

In one embodiment, the MOS capacitor further has a film of silicon oxidedisposed between the silicon substrate and the film of lanthanide oxidenanoparticles.

In one embodiment, the lanthanide oxide nanoparticles have europiumoxide (Eu₂O₃) nanoparticles or gadolinium oxide (Gd₂O₃) nanoparticles.

In another embodiment, the film of lanthanide oxide nanoparticles has athickness ranging from about 50 to about 500 nm.

In yet another embodiment, the aluminum film has a thickness of about300 nm.

In a further embodiment, the aluminum film is formed on the film oflanthanide oxide nanoparticles using electron-beam evaporation.

These and other aspects of the present invention will become apparentfrom the following description of the preferred embodiment taken inconjunction with the following drawings and their captions, althoughvariations and modifications therein may be affected without departingfrom the spirit and scope of the novel concepts of the disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows: (a) TEM image of Eu₂O₃ nanocrystals, indicating an averagediameter of about 2.4 nm; inset: electron diffraction pattern of thenanocrystals, exhibiting features arising from the {222} and {440}lattice planes; (b) absorption spectrum of the Eu₂O₃ nanocrystals,exhibiting strong absorption in UV region due to a transition from theground state to the charge-transfer state of the Eu—O bond and (inset) aweak absorption peak at 395 nm due to the 4f→4f transition; and (c)photoluminescence spectrum of the nanocrystals, exhibiting peaks arisingdue to ⁵D₀→⁷F_(J) (J=0-4) transitions.

FIG. 2 shows: (a) optical micrograph of the Eu₂O₃ film deposited on agold substrate, which appears golden in colour because of the backgroundgold substrate and the high transparency of the film; (b) EDS spectrumof the film deposited on a gold substrate, which reveals the presence ofeuropium, oxygen and carbon that originates from theoleic-acid-functionalized Eu₂O₃ nanocrystals and the gold from thesubstrate; and (c) PL spectra of the Eu₂O₃ nanocrystal films depositedon the anode and cathode. The spectra are identical to the spectrum ofthe Eu₂O₃ nanocrystals. PL spectra are shifted vertically for clarity.

FIG. 3 shows: (a) SEM image of the nanocrystal film; and (b) AFM imageof the nanocrystal film, which reveals deposition of the nanocrystalagglomerates of about 15 nm size. RMS roughness of the film determinedfrom the AFM image is 1.4 nm.

FIG. 4 shows: (a) optical micrograph of the patterned silicon substraterecorded through the nanocrystal film deposited on ITO-coated glasssubstrate, which reveals high transparency of the EPD film; and (b)transmission spectrum of a cast film of the Eu₂O₃ nanocrystals, showinghigh transparency in the visible region.

FIG. 5 shows the thickness of the EPD film as a function of the appliedvoltage for different nanocrystal concentrations. Average film thicknessis reported from the thickness measurements at different locations andstandard deviation is employed as the error bar. The large error barindicates decreased film uniformity.

FIG. 6 shows the electrophoretic mobility measurements of the EPDsuspensions with different nanocrystal concentrations.

FIG. 7 shows AFM images ((a), (b), (c), and (d)) and EDS spectrum ((e),(f), (g), and (h)) of the nanocrystal films deposited with thenanocrystal suspension concentration of 4×10¹⁵ NC cm⁻³ at the appliedvoltages of 250 V, 500 V, 750 V and 1000 V, respectively. The AFM imagesof the films reveal the agglomerate size of about 130-160 nm and RMSroughness of about 1.6-1.8 nm. The morphology and composition of thefilms were comparable.

FIG. 8 shows: (a) a schematic of the MOS capacitor structures with NCfilm as the gate oxide layer according to one embodiment of the presentinvention; (b) AFM image of the NC film (1 μm² area, height: 20 nm/div)with RMS roughness of about 1.6 nm; inset: TEM image of the about 2.4 nmdiameter Gd₂O₃ nanocrystals (Image: 6 nm×6 nm); (c) EDS spectrum of theNC film; and (d) SEM image (top view) of the MOS capacitor structures.

FIG. 9 shows C-V characteristics of the MOS capacitors, fabricated fromNC films that were deposited on the anode and cathode according to oneembodiment of the present invention. The thickness of the film was 116nm±10 nm, and the average area of the capacitors was 1.96×10⁵ μm².

FIG. 10 shows C-V characteristics of the MOS capacitors with differentthicknesses of the NC films (oxide layer) according to one embodiment ofthe present invention.

FIG. 11 shows a graph of capacitance versus inverse of NC film (oxidelayer) thickness for four different MOS capacitors according to oneembodiment of the present invention. The slope of the linear regressionfit was proportional to the permittivity of the nanocrystal film and,hence, to the film's dielectric constant, κ=3.90.

DETAILED DESCRIPTION

The present invention is more particularly described in the followingexamples that are intended as illustrative only since numerousmodifications and variations therein will be apparent to those skilledin the art. Various embodiments of the invention are now described indetail. Referring to the drawings, FIGS. 1-11, like numbers, if any,indicate like components throughout the views. As used in thedescription herein and throughout the claims that follow, the meaning of“a”, “an”, and “the” includes plural reference unless the contextclearly dictates otherwise. Also, as used in the description herein andthroughout the claims that follow, the meaning of “in” includes “in” and“on” unless the context clearly dictates otherwise. Moreover, titles orsubtitles may be used in the specification for the convenience of areader, which shall have no influence on the scope of the presentinvention. Additionally, some terms used in this specification are morespecifically defined below.

DEFINITIONS

The terms used in this specification generally have their ordinarymeanings in the art, within the context of the invention, and in thespecific context where each term is used. Certain terms that are used todescribe the invention are discussed below, or elsewhere in thespecification, to provide additional guidance to the practitionerregarding the description of the invention. For convenience, certainterms may be highlighted, for example using italics and/or quotationmarks. The use of highlighting has no influence on the scope and meaningof a term; the scope and meaning of a term is the same, in the samecontext, whether or not it is highlighted. It will be appreciated thatsame thing can be said in more than one way. Consequently, alternativelanguage and synonyms may be used for any one or more of the termsdiscussed herein, nor is any special significance to be placed uponwhether or not a term is elaborated or discussed herein. Synonyms forcertain terms are provided. A recital of one or more synonyms does notexclude the use of other synonyms. The use of examples anywhere in thisspecification including examples of any terms discussed herein isillustrative only, and in no way limits the scope and meaning of theinvention or of any exemplified term. Likewise, the invention is notlimited to various embodiments given in this specification.

Unless otherwise defined, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which this invention pertains. In the case of conflict, thepresent document, including definitions will control.

As used herein, “around”, “about” or “approximately” shall generallymean within 20 percent, preferably within 10 percent, and morepreferably within 5 percent of a given value or range. Numericalquantities given herein are approximate, meaning that the term “around”,“about” or “approximately” can be inferred if not expressly stated.

As used herein, if any, the term “atomic force microscope (AFM)” orscanning force microscope (SFM) refers to a very high-resolution type ofscanning probe microscope, with demonstrated resolution of fractions ofa nanometer, more than 1000 times better than the optical diffractionlimit. The term “microscope” in the name of “AFM” is actually a misnomerbecause it implies looking, while in fact the information is gathered orthe action is taken by “feeling” the surface with a mechanical probe.The AFM in general has a microscale cantilever with a tip portion(probe) at its end that is used to scan the specimen surface. Thecantilever is typically silicon or silicon nitride with a tip radius ofcurvature on the order of nanometers. When the tip is brought intoproximity of a sample surface, forces between the tip and the samplelead to a deflection of the cantilever according to Hooke's law. The AFMcan be utilized in a variety of applications.

As used herein, if any, the term “transmission electron microscopy(TEM)” refers to a microscopy technique whereby a beam of electrons istransmitted through an ultra thin specimen, interacting with thespecimen as it passes through it. An image is formed from the electronstransmitted through the specimen, magnified and focused by an objectivelens and appears on an imaging screen, a fluorescent screen in mostTEMs, plus a monitor, or on a layer of photographic film, or to bedetected by a sensor such as a CCD camera.

As used herein, if any, the term “scanning electron microscope (SEM)”refers to a type of electron microscope that images the sample surfaceby scanning it with a high-energy beam of electrons in a raster scanpattern. The electrons interact with the atoms that make up the sampleproducing signals that contain information about the sample's surfacetopography, composition and other properties such as electricalconductivity.

As used herein, if any, the term “energy dispersive X-ray spectroscopy(EDS or EDX)” refers to an analytical technique used for the elementalanalysis or chemical characterization of a sample. It is one of thevariants of X-ray fluorescence spectroscopy which analyzes X-raysemitted by the matter in response to being hit with charged particlessuch as electrons or protons, or a beam of X-rays. Its characterizationcapabilities are due in large part to the fundamental principle thateach element has a unique atomic structure allowing X-rays that arecharacteristic of an element's atomic structure to be identifieduniquely from one another.

As used herein, if any, the term “absorption spectroscopy” refers tospectroscopic techniques that measure the absorption of radiation, as afunction of frequency or wavelength, due to its interaction with asample. The sample absorbs energy, i.e., photons, from the radiatingfield. The intensity of the absorption varies as a function offrequency, and this variation is the absorption spectrum. Absorptionspectroscopy is employed as an analytical chemistry tool to determinethe presence of a particular substance in a sample and, in many cases,to quantify the amount of the substance present. Infrared andultraviolet-visible (UV-Vis) spectroscopy are particularly common inanalytical applications. The term “infrared spectroscopy” refers toabsorption spectroscopy in the infrared spectral region; and the term“ultraviolet-visible (UV-Vis) spectroscopy” refers to absorptionspectroscopy in the ultraviolet-visible spectral region.

As used herein, if any, the term “photoluminescence spectroscopy” refersto a contactless, nondestructive method of probing the electronicstructure of materials. In this method, light is directed onto a sample,where it is absorbed and imparts excess energy into the material in aprocess called photo-excitation. One way this excess energy can bedissipated by the sample is through the emission of light, orluminescence. In the case of photo-excitation, this luminescence iscalled photoluminescence. The intensity and spectral content of thisphotoluminescence is a direct measure of various important materialproperties.

As used herein, if any, the term “C-V”, as in C-V profiling, or C-Vmeasurements, or C-V characteristics, stands for capacitance-voltage,and refers to a technique used for characterization of semiconductormaterials and devices. The technique uses a metal-semiconductor junction(Schottky barrier) or a p-n junction or a MOSFET to create a depletionregion, a region which is empty of conducting electrons and holes, butmay contain ionized donors and electrically active defects or traps. Thedepletion region with its ionized charges inside behaves like acapacitor. By varying the voltage applied to the junction it is possibleto vary the depletion width. The dependence of the depletion width uponthe applied voltage provides information on the semiconductor's internalcharacteristics, such as its doping profile and electrically activedefect densities. Measurements may be done at DC, or using both DC and asmall-signal AC signal (the conductance method), or using a large-signaltransient voltage.

As used herein, the term “ITO” or “ITO glass” refers to indium tinoxide, or tin-doped indium oxide, which is a solid solution ofindium(III) oxide (In₂O₃) and tin(IV) oxide (SnO₂), typically 90% In₂O₃,10% SnO₂ by weight. It is transparent and colorless in thin layers whilein bulk form it is yellowish to grey. In the infrared region of thespectrum it is a metal-like mirror. Indium tin oxide is one of the mostwidely used transparent conducting oxides and so has main feature of acombination of electrical conductivity and optical transparency. Thinfilms of indium tin oxide are most commonly deposited on surfaces byelectron beam evaporation, physical vapor deposition, or a range ofsputter deposition techniques.

As used herein, “nanoscopic-scale,” “nanoscopic,” “nanometer-scale,”“nanoscale,” “nanocomposites,” “nanoparticles,” the “nano-” prefix, andthe like generally refers to elements or articles having widths ordiameters of less than about 1 μm, preferably less than about 100 nm insome cases. In all embodiments, specified widths can be smallest width(i.e. a width as specified where, at that location, the article can havea larger width in a different dimension), or largest width (i.e. where,at that location, the article's width is no wider than as specified, butcan have a length that is greater).

As used herein, “plurality” means two or more.

As used herein, the terms “comprising,” “including,” “carrying,”“having,” “containing,” “involving,” and the like are to be understoodto be open-ended, i.e., to mean including but not limited to.

OVERVIEW OF THE INVENTION

Lanthanide oxides such as europium oxide (Eu₂O₃) and gadolinium oxide(Gd₂O₃) are known for their light emitting and high-κ dielectricproperties, respectively [76, 58, 77]. The Eu³⁺-doped Gd₂O₃, inmicrocrystalline form, has been employed in video displays and tri-colorfluorescent lamps as a red phosphor [78]. Recently, nanocrystalline formof Eu³⁺-doped sesquioxides has gained research interest due to theirpotential use in luminescent biological tags, efficient light emittingdevices, and high-resolution displays. Gd₂O₃ has received researchattention because of its high-κ dielectric properties. Gd₂O₃ has beenproposed as silicon dioxide replacement for gate oxide in ultra-smallcomplementary metal-oxide-semiconductor (CMOS) devices [77]. Mostapplications of luminescent and dielectric materials require theirimplementation in thin-film form. The Eu₂O₃ and Gd₂O₃ nanocrystals, madevia colloidal techniques, need to be assembled into thin-film form tostudy their optical and dielectric properties. Of the thin-filmdeposition techniques, electrophoretic deposition (EPD) is the promisingtechnique to deposit nanomaterials. EPD offers a simple design set-upand provides substantial thickness control at high deposition rates toassemble particles site-selectively of any size and shape.

In one embodiment of the present invention, EPD technique is employed todeposit ultra-small (<3 nm) colloidal lanthanide oxide nanoparticles,specifically Eu₂O₃ and Gd₂O₃ nanocrystals, to form uniform, homogeneousfilms. The nanocrystals were synthesized via hot solution phase methodand purified with ethanol prior to deposition [71]. The films weredeposited onto conducting substrates such as gold-coated glass,gold-coated silicon, stainless steel (316L), indium-tin-oxide-coated(ITO) glass, and p-type silicon from a suspension of the nanocrystals inhexane.

A typical EPD involves the following sequence of the steps: applicationof a DC voltage to a pair of electrodes, insertion of the pair ofelectrodes into the EPD suspension (area: about 18 mm×13 mm), depositionfor 15 min, and extraction of the pair of electrodes from thesuspension, and drying in air for 5 min while maintaining the appliedvoltage. The films of different thicknesses were deposited employingdifferent nanocrystal suspension concentrations (about 1-10×10¹⁵ NC/cc)and different applied voltages (about 250-1000 V).

Thus, in one aspect, the present invention provides a method of forminga film of lanthanide oxide nanoparticles. In one embodiment, the methodcomprises the steps of: (a) providing a first substrate with aconducting surface and a second substrate that is positioned apart fromthe first substrate, (b) applying a voltage between the first substrateand the second substrate, (c) immersing the first substrate and thesecond substrate in a solution that comprises a plurality of lanthanideoxide nanoparticles suspended in a non-polar solvent or apolar solventfor a first duration of time effective to form a film of lanthanideoxide nanoparticles on the conducting surface of the first substrate,and (d) after the immersing step, removing the first substrate from thesolution and exposing the first substrate to air while maintaining theapplied voltage for a second duration of time to dry the film oflanthanide oxide nanoparticles formed on the conducting surface of thefirst substrate.

In one embodiment, the first substrate is gold-coated glass, gold-coatedsilicon, stainless steel (316L), indium-tin-oxide (ITO)-coated glass, ordoped silicon.

In one embodiment, the applied voltage, V, is in the range of 0volts<V≦1000 volts.

In one embodiment, the non-polar solvent or apolar solvent includes atleast one of hexane, octane and mixtures thereof, and each of the firstduration of time, T1 and the second duration of time voltage, T2, is inthe range of 0 minutes<T1, T2≦30 minutes.

In one embodiment, the first duration of time, T1 and the secondduration of time voltage, T2, can be same or different.

In one embodiment, the solution has a concentration ranging from about1×10¹⁴ nanoparticles per cubic centimeter to about 10×10¹⁵ nanoparticlesper cubic centimeter.

In one embodiment, the film of lanthanide oxide nanoparticles formed onthe conducting surface of the first substrate has a thickness rangingfrom about 50 to about 500 nm.

In one embodiment, the film of lanthanide oxide nanoparticles formed onthe conducting surface of the first substrate comprises randomlyclose-packed lanthanide oxide nanoparticles with a packing density ofabout 66%.

In one embodiment, the lanthanide oxide nanoparticles are europium oxide(Eu₂O₃) nanoparticles or gadolinium oxide (Gd₂O₃) nanoparticles.

In yet another embodiment, the lanthanide oxide nanoparticles have acore diameter ranging from about 2 to about 3 nm.

In a further embodiment, the lanthanide oxide nanoparticles aresurface-passivated with oleic acid.

In another aspect, the present invention provides an article ofmanufacture having a film of the lanthanide oxide nanoparticles made bythe method set forth immediately above.

In yet another aspect, the present invention provides ametal-oxide-semiconductor (MOS) capacitor. In one embodiment, the MOScapacitor has: (a) a silicon substrate having a first surface, (b) afilm of lanthanide oxide nanoparticles formed on the first surface ofthe silicon substrate using the method set forth immediately above, and(c) an aluminum film formed on the film of lanthanide oxidenanoparticles, wherein the film of lanthanide oxide nanoparticlescomprises randomly close-packed lanthanide oxide nanoparticles with apacking density of about 66%.

In one embodiment, the silicon substrate has p-type silicon.

In one embodiment, the first surface of the silicon substrate is ap-(100) surface of silicon.

In one embodiment, the MOS capacitor further has a film of silicon oxidedisposed between the silicon substrate and the film of lanthanide oxidenanoparticles.

In one embodiment, the lanthanide oxide nanoparticles include europiumoxide (Eu₂O₃) nanoparticles or gadolinium oxide (Gd₂O₃) nanoparticles.

In another embodiment, the film of lanthanide oxide nanoparticles has athickness ranging from about 50 to about 500 nm.

In yet another embodiment, the aluminum film has a thickness of about300 nm.

In a further embodiment, the aluminum film is formed on the film oflanthanide oxide nanoparticles using electron-beam evaporation.

Additional details are set forth below.

EXAMPLES

Aspects of the present teachings may be further understood in light ofthe following examples, which should not be construed as limiting thescope of the present teachings in any way.

Example 1

Eu₂O₃ Nanocrystal Films.

The controlled assembly of nanomaterials into microscopic andmacroscopic structures is one of the most important and continuouslygrowing research directions in nanotechnology. Efficient bottom-upassembly approaches are essential to the development of next-generationoptical, magnetic and electronic devices that utilize the uniqueproperties of metallic, semiconducting or insulating nanomaterials.Currently employed nanomaterial assembly techniques includedrop-casting, spin-casting [1], self-assembly [2-4], Langmuir-Blodgett[5, 6] and electrophoretic deposition (EPD) [7-9]. For an assemblytechnique to be commercially or industrially viable for the fabricationof nanostructured devices, the technique must involve flexibility of thetype of material (metal/semiconductor/insulator), superior filmthickness control, high rate of assembly and site-selectivity. Of theaforementioned deposition techniques, electrophoretic deposition isarguably the most promising for nanomaterial deposition, as EPD offers asimple design set-up and provides substantial thickness control at rapiddeposition rates to assemble site-selectively particles of any size,shape and type [10]. EPD has been employed successfully to deposit filmsof metallic (Au, Pt) [11, 12], semiconducting (CdSe, ZnO) [7, 13],insulating (TiO₂, SiO₂, Eu₂O₃) [14-18] and magnetic (Fe₃O₄, Fe₂O₃) [9,19] nanocrystals. Other types of nanomaterials, such as polymernanoparticles [20, 21] and carbon nanotubes (CNTs) [9, 22-27], have beenassembled_via EPD. Homogeneous and smooth films of nanocrystals havebeen reported for the nanocrystals functionalized with surface cappingligands such as CdSe, Fe₂O₃, Fe₃O₄ and Eu₂O_(3 [)7, 9, 16, 28]. Suchnanocrystals form stable suspensions in non-polar solvents such ashexane because of the hydrophobic surface capping ligands, often used incolloidal nanocrystal syntheses to stabilize the surface of thenanocrystals [29-31]. Surface-stabilized Eu₂O₃ nanocrystal films havebeen of recent interest because of their strong ultraviolet (UV)absorption and the characteristic red colour emission of thenanocrystals [31]. Eu₂O₃ nanocrystal films have potential applicationsin UV absorption coatings, photoactive coatings and fluorescent screens[32, 33]. In these applications, the deposition of transparent filmsfacilitates the efficient transmission of visible light. Since Eu₂O₃nanocrystals have weak absorption in the visible, implementation ofsmall-diameter, light-emitting nanocrystals should minimize lightscattering losses and thereby enhance the transmission of lightthroughout the visible spectrum.

The relationship among the film thickness, the EPD suspension parameters(particle mobility) and the EPD process parameters (applied voltage,deposition time and nanocrystal concentration) was originally studied byHamaker [34]. Later, the effect of these parameters on the deposition ofvarious micron-sized ceramic particles (Al₂O₃, TiO₂, SiO₂) from theirsuspension in water and organic polar solvents was investigated [14, 15,23, 35-40]. In contrast, research exploring the deposition ofnanocrystals that are suspended in non-polar organic solvents (hexane)is relatively limited [7, 9, 19, 41]. Since the EPD suspensionproperties are dependent on the suspension medium, the properties fornanocrystals suspended in non-polar solvents differ from those ofparticles suspended in water and organic polar solvents. Nanocrystalswith surface capping ligands form stable suspension in nonpolar solvent(e.g. hexane) because of the steric stabilization of the nanocrystalsurface with ligands. Steric repulsive forces, developed between thenanocrystals by the ligands, overcome the van der Waals attractionforces to form the stable suspension [42]. The origin of surface chargesin sterically stabilized nanocrystal suspensions also differs from thatof electrostatically stabilized particles in suspension. Inelectrostatically stabilized systems, surface charges develop because ofpolar solvent molecules and free ions in the solvent [43, 44]. Incontrast, the charges on the larger nanocrystals may originate from theadsorption of uncharged ligands, ion exchange between the adsorbedligand and the surface, and desorption of the ionized ligands [45-47].Thermal charging of nanocrystals in suspension has been another debatedorigin of charge [28, 48]. However, our most substantive concernregarding surface charge arises from charge tuning on the nanocrystalsachieved through the addition of ligands and/or removal of ligands viapurification steps [2]. Since a fraction of the ligands is detachedduring each step of nanocrystal purification, the nanocrystal surfacecharge can become changed. The effect of the number of nanocrystalpurification steps on the quality of electrophoretically deposited CdSenanocrystal films has been reported, which provides a considerableinsight into the optimization of film quality [41]. Although studieshave been conducted on the effect of EPD process parameters on thequality of films composed of micron-sized (or larger) electrostaticallystabilized particles [49, 50], no specific report, to date, existsexploring the effect of EPD process parameters on the quality ofsterically stabilized or electrostatically stabilized nanocrystallinefilms.

In various embodiments of the present invention, certain transparentfilms of Eu₂O₃ nanocrystals were fabricated via electrophoreticdeposition and investigated the effect of EPD processing parameters(applied voltage, deposition time and nanocrystal concentration) on theuniformity of the films. About 2.4 nm Eu₂O₃ nanocrystals that werecapped with oleic acid and were suspended in a non-polar solvent orapolar solvent such as hexane were employed. The films were depositedonto two types of substrates: gold-coated glass and indium-tin-oxide(ITO)-coated glass. The deposition of the nanocrystals on the anode andcathode was confirmed by conducting elemental analysis via energydispersive spectroscopy (EDS) and optical analysis usingphotoluminescence spectroscopy (PL). Scanning electron microscopy (SEM)and atomic force microscopy (AFM) confirmed the deposition ofhomogeneous, topographically smooth films that were composed of denselypacked agglomerates (about 15 nm) of the Eu₂O₃ nanocrystals.Additionally, the effect of EPD processing parameters on the thicknessand uniformity of the transparent films were explored. The effect thatthese parameters had on the thickness homogeneity across the filmprovided a marked insight into the growth mechanisms of the films.

Nanocrystal Synthesis and Purification.

Europium oxide (Eu₂O₃) nanocrystals were synthesized via a two-stagesolution-phase technique, as described in detail by the inventorselsewhere [31]. In the first stage, europium oleate was synthesized byheating a mixture of europium (III) chloride hexahydrate (EuCl₃.6H₂O)and sodium oleate (CH₃(CH₂)₇CH:CH(CH₂)₇COONa) at 70° C. for 4 hours in ahexane-ethanol-water mixture. Next, a mixture of europium oleate (0.5mM) and oleic acid (0.25 mM) was heated to 350° C. in tri-n-octylamine(7 ml) and maintained at that temperature for an hour during the secondstage. This synthesis yielded Eu₂O₃ nanocrystals of 2.4±0.3 nm corediameter, surface-passivated with oleic acid. The nanocrystals wereisolated from the reaction mixture by a purification process thatinvolved sequential precipitation and centrifugation sequences. Theaddition of ethanol to the reaction mixture facilitated nanocrystalprecipitation; centrifugation helped to isolate the nanocrystals. Theisolated nanocrystals were dispersed back into hexane and theprecipitation-centrifugation sequence was repeated. The nanocrystals,purified four times (4×) by this procedure, were employed fortransmission electron microscopy (TEM) imaging. The nanocrystals werepurified further (10×) to deposit the optimum quality film. The finalnanocrystal suspensions for employment in electrophoretic depositionwere prepared in hexane.

Electrophoretic Deposition (EPD).

The EPD technique was used to deposit Eu₂O₃ nanocrystals onto two typesof substrate: gold-coated glass (gold electrode) andindium-tin-oxide-coated glass (ITO electrode). Gold electrodes werefabricated by the thermal evaporation of about 20 nm of chromium, usedas an adhesion layer, onto a glass substrate followed by about 125 nm ofgold. ITO electrodes (surface-resistant: 15-25Ω) were purchased fromDelta Technologies, Ltd. The electrodes were cut to the size of 25 mm×13mm for EPD. Then, the electrodes were cleaned sequentially usingacetone, ethanol and hexane with an intermediate drying step using astream of nitrogen. The electrodes were mounted in a verticalparallel-plate configuration with a gap of about 5 mm. For a single EPD,a pair of the same type of substrate was employed as the positive andnegative electrodes. A Keithley 6517A electrometer was utilized to applyDC bias to the electrodes and to measure current flowing through thesuspension during the deposition. A typical EPD involved the followingsequence: application of a DC voltage, insertion of an electrode pairinto the EPD suspension (deposition area: about 18 mm×13 mm), depositionfor 15 minutes, and extraction of the electrode pair from the suspensionand drying in air for 5 minutes while maintaining the applied voltage.

Characterization.

The size of the Eu₂O₃ nanocrystals was measured from the image of thenanocrystals, acquired with a Philips CM 20 transmission electronmicroscope. The absorption and photoluminescence spectra of thenanocrystal suspensions were recorded using a Cary 5000spectrophotometer and a Fluorolog-3-FL3-111 spectrophotofluorometer,respectively. Electrophoretic mobility of the nanocrystals was measuredin suspension (hexane) from dynamic light scattering (DLS) experiments,performed on a Malvern Nano ZS system. Optical micrographs of thenanocrystal films were acquired with a Leitz microscope connected to aCFM-USB-2 camera from Angstrom Sun Technologies Inc. Surfacemorphologies of the films were analysed using a Hitachi S-4200 scanningelectron microscope and a Digital Instruments Nanoscope III atomic forcemicroscope in tapping mode. Deposition of the nanocrystals on theelectrodes was confirmed by performing elemental analysis of theelectrodes using energy dispersive spectroscopy with a Link ISIS Series300 microanalysis system (Oxford Instruments). Film thickness using aVeeco Dektak 150 surface profiler was measured. The transmission andphotoluminescence spectra of the nanocrystal films were acquired using aCary 5000 spectrophotometer and a Fluorolog-3-FL3-111spectrophotofluorometer, respectively.

Results and Implications.

FIG. 1 shows a TEM image and electron diffraction pattern of the Eu₂O₃nanocrystals, and absorption and PL spectra of the nanocrystals thatwere dispersed in hexane. The average core diameter of the Eu₂O₃nanocrystals was d_(NC)=2.4±0.3 nm, confirmed with the TEM image (asshown in FIG. 1( a). Shown as an inset to FIG. 1( a), an electrondiffraction pattern of the nanocrystals revealed features attributableto the {222} and {440} lattice planes of Eu₂O₃. FIG. 1( b) shows theabsorption spectrum of the nanocrystals. The nanocrystals exhibitedstrong absorption in the ultraviolet (UV) region but showed weakabsorption in the visible region. The strong absorption peak at 225 nmwas attributed to the transition between the ground state and thecharge-transfer state of the Eu—O bond (4f⁷→4f⁷2p⁻¹) [51-53]. Inaddition, the weak absorption peak at 395 nm, shown in the inset to FIG.1( b), arose from the 4f→4f transition [51]. FIG. 1( c) shows thephotoluminescence spectrum of the nanocrystals upon UV excitation (254nm). The peaks corresponded to the ⁵D₀→⁷F_(J) (J=0-4) radiative energytransitions within Eu³⁺ ions [31]. Of these characteristic transitions,the ⁵D₀→⁷F₂ is the most sensitive transition to the location of the Eu³⁺ion within the crystal. The most intense PL peaks (612, 620 and 624 nm)were observed [31].

Known for its production of high quality films of colloidalnanocrystals, EPD was implemented to deposit the Eu₂O₃ nanocrystals fromtheir suspension in hexane [9, 54]. Nanocrystal films were deposited ongold and ITO electrodes. FIG. 2( a) shows a typical optical micrographof a Eu₂O₃ nanocrystal film, deposited on a gold electrode (anode). Thenanocrystal film was cast with an applied voltage of 250 V and ananocrystal concentration of 2×10¹⁵ NC cm⁻³. The yellowish colour of thefilm is due to its high transparency and the colour of the underlyinggold substrate. The film was continuous with no visible defects largerthan 5 μm. The film that was deposited on the cathode had a comparableappearance. The thickness of the film was about 110 nm, which wasmeasured using surface profilometry. To verify deposition of Eu₂O₃nanocrystals on the electrodes, EDS was performed for elemental analysisand PL for optical analysis. FIG. 2( b) shows the EDS spectrum of thenanocrystal film, deposited on the anode. The presence of europium,oxygen and carbon peaks confirmed the deposition of theoleic-acid-functionalized Eu₂O₃ nanocrystals. Also, gold was detectedbecause of the underlying gold substrate. Similarly, deposition of theEu₂O₃ nanocrystals was confirmed on the cathode. FIG. 2( c) shows the PLspectra of the anode and cathode upon UV excitation (254 nm). Thespectra exhibited all of the peaks corresponding to the ⁵D₀→⁷F_(J)(J=0-4) energy transitions of the Eu³⁺ ion. The spectra were identicalto the spectrum of the Eu₂O₃ nanocrystals (as shown in FIG. 1( c),confirming deposition of the Eu₂O₃ nanocrystals. Thus, the EDS and PLspectra of cathode and anode confirmed deposition of the Eu₂O₃nanocrystal film.

Surface morphology of the nanocrystal film was probed with SEM and AFM.FIG. 3(a) shows the SEM image of the nanocrystal film, deposited on theanode. The nanocrystal film was topographically smooth and uniform. Thefilm on the cathode had comparable surface morphology. AFM was employedto perform high-resolution surface topological analysis of thenanocrystal film. The AFM image, shown in FIG. 3( b), revealed that thefilm was composed of agglomerates of the Eu₂O₃ nanocrystals,approximately 15-20 nm in diameter. The apparent deposition ofagglomerates of the nanocrystals instead of individual nanocrystalsmotivated us to identify the formation of any agglomerates in EPDsuspension prior to the deposition. New TEM samples were prepared forimaging by drop-casting the EPD suspensions onto the grids. These newsamples confirmed the absence of any agglomerates of the Eu₂O₃nanocrystals in the EPD suspension. Thus, the agglomeration of thenanocrystals likely occurred under the influence of the electric fieldduring EPD. The agglomeration may have occurred at one or more of thefollowing stages: (a) immediate agglomeration upon application of thevoltage to the electrodes in the EPD suspension; (b) agglomeration nearthe electrodes following an increase in the nanocrystal concentrationdue to movement of charged nanocrystals towards the respectiveelectrodes and (c) reorganization of the deposited nanocrystals at theelectrode leading to agglomeration. Even though the nanocrystalsagglomerated under the influence of an electric field, the extent ofagglomeration was limited because of sufficient ligand coverage on thenanocrystal surface. The deposited agglomerates packed close to eachother, forming a continuous and densely arranged film (as shown in FIG.3( b). It was reported a packing fraction of about 0.63 for random closepacking, also known as glassy packing, of spheres [55]. The observedpacking fraction (about 0.56) by practicing the present invention wascalculated on the basis of the AFM image and was within the lower regimeof glassy packing Nonetheless, these films were particularly smooth. Theroot mean square (RMS) surface roughness, determined from the AFM imageof the film, was about 1.4 nm, which was smaller than the diameter ofone nanocrystal. The two plausible reasons for the high smoothness ofthe films are: (a) a small fraction of the nanocrystals was depositedalong with the agglomerates of the nanocrystals and (b) the agglomeratesof the nanocrystals had a degree of structural flexibility since thenanocrystal surface was partially covered with ‘soft’ surface cappingligands. Thus, SEM and AFM imaging confirmed the formation of a smooth,uniform and densely packed film of the agglomerates of the Eu₂O₃nanocrystals.

To demonstrate transparency of the Eu₂O₃ nanocrystal film, an opticalmicrograph of the patterned silicon substrate was recorded through thenanocrystal film that was deposited on the ITO electrode (as shown inFIG. 4( a). The patterned substrate was clearly visible, which confirmedthe formation of highly transparent film. Transmission spectroscopy wasperformed on the same film to determine its transmission properties inthe visible region. Intensity oscillations of transmitted light wereseen in the visible transmission spectrum, which were due to Bragginterference because of the thickness of the film (about 110 nm thick).To measure the transmission of the film, an about 500 μm film wasdeposited using a drop-cast technique. FIG. 4( b) shows the transmissionspectrum of the drop-cast film, which reveals high transparency in thevisible region (>80%). Considering the high transparency of the thickfilm, the electrophoretically deposited thin film should have acomparable transmission. High transparency of the film was achieved byminimizing the scattering loss of visible light within the nanocrystalfilm. The intensity of scattered light off a nanoparticle within thevisible region is best expressed by the Rayleigh scattering equation,which is appropriate within the limit x=πd_(NC)/λ, x<<1. For the averagesize (15 nm) of agglomerates of the Eu₂O₃ nanocrystals, x ranges between0.12 and 0.06 in the visible spectral region. Since the nanocrystalfilms had a transparent appearance instead of hazy (e.g. translucent),the use of Rayleigh scattering theory instead of Mie scattering theoryis valid. Rayleigh scattering intensity per particle, I_(S), is writtenas

$\begin{matrix}{I_{S} = {\left( \frac{2\pi}{\lambda} \right)^{4}\left( \frac{n^{2} - 1}{n^{2} + 2} \right)^{2}\left( \frac{1 + {\cos^{2}\theta}}{2R^{2}} \right)\left( \frac{d_{NC}}{2} \right)^{6}I_{0}}} & (1)\end{matrix}$where λ is the wavelength of the incident light, n is the refractiveindex of the particle, θ is the scattering angle, R is the distance tothe particle from the point of observation, d_(NC) is the particlediameter and I₀ is the intensity of the incident light. Clearly, thesmall size of the Eu₂O₃ nanocrystal agglomerates within the filmfacilitated reduction of the scattering loss of visible light becausethe scattering intensity is proportional to the sixth power of theparticle size. Thus, the small size of the Eu₂O₃ nanocrystalagglomerates was the key to achieving highly transparent films.

The ability to control the thickness of the films made according to thevarious embodiments of the present invention is extremely importantwhile maintaining film quality. The Hamaker equation (2) correlates theamount of particles deposited (deposit yield, w) during EPD to theelectrophoretic mobility (μ), the electric field (E), the electrode area(A), the particle mass concentration in the suspension (C) and thedeposition time (t):w=μEACt  (2)The electrophoretic mobility (μ) of the particles, suspended inlow-dielectric solvents, is related to the zeta potential of theparticle (ζ), the solvent viscosity (η), relative permittivity of thesolvent (∈_(r)) and the permittivity of vacuum (∈₀) of the suspensionthrough the Hückel equation (3):

$\begin{matrix}{\mu = \frac{2ɛ_{0}ɛ_{r}\zeta}{3\eta}} & (3)\end{matrix}$

The deposition yield, as stated by the Hamaker equation, is written as

$\begin{matrix}{w = \frac{2ɛ_{0}ɛ_{r}\zeta\; E\; A\; C\; t}{3\eta}} & (4)\end{matrix}$

Since the solvent (hexane) and electrode set-up (deposition area, A=18mm×13 mm, 5 mm gap) for EPD were the same, the parameters (∈_(r), η andA) remained constant. The number of purification steps, employed topurify the nanocrystals, affected the coverage of surface cappingligands on the nanocrystals. Net charges possessed by the nanocrystalsin solution are related to the coverage of ligands on the nanocrystalsurface (steric stabilization). Hence, the Eu₂O₃ nanocrystals insolution were purified by the same process to maintain a similar zetapotential of the nanocrystals for all EPDs. Thus, the deposition yield(and film thickness) can be controlled via the EPD process parameterssuch as the electric field (E), the particle concentration (C) and thedeposition time (t). For a deposition sequence with a constant appliedvoltage and a fixed initial concentration, the deposition rate decreasesas the deposition time increases [56]. A decreasing particleconcentration within the EPD suspension and an increasing voltage dropacross the growing film of insulating/semiconducting nanoparticles alsodecreases the deposition rate for extended deposition times. It wasreported that the current density and deposition rate of hydroxyapatitedecreased as a function of deposition time [57]. A decreasing currentdensity through the particle suspension is an indication of a decreasingdeposition rate. It has been observed that the current density throughthe Eu₂O₃ nanocrystal suspension prepared according to variousembodiments of the present invention dropped at least 80% within tenminutes of the beginning of the deposition run. Since the depositionrate was expected to be low at times beyond fifteen minutes ofdeposition time, the deposition time fixed at 15 minutes was maintainedfor all EPD experiments and the applied voltage and the nanocrystalconcentration was varied to monitor the uniformity and thickness of thefilms.

EPD of the Eu₂O₃ nanocrystals was performed at different appliedvoltages (250, 500, 750 and 1000 V) and with different nanocrystalconcentrations (1×10¹⁵, 2×10¹⁵ and 4×10¹⁵ NC cm⁻³) to understand theireffect on the thickness and uniformity of the nanocrystal film.Thickness measurements were conducted at five locations on threedifferent samples, and the average thickness was determined with thestandard deviation of the thicknesses as the error bar. Hence, the errorbar conveys the thickness uniformity of the film. FIG. 5 shows the graphof the nanocrystal film thickness as a function of the applied voltagefor different nanocrystal concentrations. The film thickness increasedas a function of the applied voltage and nanocrystal concentration, asexpected. When the applied voltage was increased, more nanocrystalsmoved toward the electrodes under the influence of increased electricfield and deposited to form films. Similarly, when the nanocrystalconcentration was increased, more charged nanocrystals were availablefor deposition, which led to the formation of thicker films. Byperforming electrophoretic mobility measurements on the EPD suspensionsof different nanocrystal concentrations, it was confirmed that morecharged nanocrystals were available for deposition as the nanocrystalconcentration increased. FIG. 6 shows that the scattering intensity ofthe particles increased with nanocrystal concentration for a givenelectrophoretic mobility. Subsequently, the thickness of the nanocrystalfilm increased with the EPD process parameters (applied voltage andnanocrystal concentration). During EPD, a constant applied voltage wasmaintained, but the nanocrystal concentration of the EPD suspensiondecreased with time as the Eu₂O₃ nanocrystal film grew. The growth ofthe film slowed as the EPD progressed. The two factors that slowed downthe growth were: (a) the increasing voltage drop across the growing filmof the insulating Eu₂O₃ nanocrystals and (b) the depletion of chargednanocrystals from the EPD suspension. Since voltage drop across the filmincreased as the film grew, the effective voltage across the EPDsuspension decreased because the applied voltage was constant. For agiven nanocrystal concentration, thicker nanocrystal films weredeposited when higher applied voltages were employed. The application ofhigher voltage between the electrodes facilitated an increased effectivevoltage across the EPD suspension, resulting in thicker films. Sincethicker films were deposited with higher applied voltages for a givennanocrystal concentration, the nanocrystal suspension was not entirelydepleted of charged nanocrystals. Thus, the increasing voltage dropacross the growing Eu₂O₃ nanocrystal film was primarily responsible forrestricting growth of the film.

EPD process parameters (applied voltage and nanocrystal concentration)altered the uniformity of the Eu₂O₃ nanocrystal film as illustrated inFIG. 5). For a given nanocrystal concentration, film uniformitydecreased (larger error bar) as the applied voltage increased. Also, thethickness uniformity decreased (larger error bar) for higher nanocrystalconcentrations for a given applied voltage. Although the films wereincreasingly non-uniform, we observed a particular pattern in thicknessvariation. The films were thick towards the edges of the electrode andwere thin in the centre of the electrode, which suggested the presenceof strong fringe electric fields near the edges of the electrode.Naturally the fringe field increased with applied voltage; therefore,more nanocrystals deposited near the edges of the electrode, increasingthe non-uniformity of the film. Also, the non-uniformity of the filmincreased when the nanocrystal concentration increased. Since the sameEPD setup (deposition area: 18 mm×13 mm electrode, 5 mm gap) wasemployed for all the depositions, variation in the thickness uniformityof the film was purely a result of changes in the EPD processparameters. Thus, the nanocrystal concentration and the applied voltagecannot be increased indefinitely to increase deposition rate or filmthickness because they affect the uniformity of the EPD film.

The microscopic morphology and the elemental composition of the EPDfilms formed according to various embodiments of the present inventionwere analysed as a function of film thickness, which explained whetherthe nanocrystals underwent any chemical, geometrical or topologicalmodifications while being deposited under the EPD electric field. It waschosen to interrogate films produced from the highest nanocrystalconcentration, 4×10¹⁵ NC cm⁻³, according to one embodiment of thepresent invention, as they yielded the most substantial deviations fromuniformity in the topology, substrate coverage and roughness in thefilms when assessed at the macroscopic level. It was surmised that suchfilm characteristics would yield the largest changes inmicroscopic/nanoscale topology, morphology and compositional changes, ifany such change existed. AFM imaging and EDS analysis were performed onthe four films with different thicknesses, which were deposited at fourdifferent applied voltages formed according to various embodiments ofthe present invention. FIGS. 7( a)-(d) show AFM images of the EPD filmsdeposited at 250 V, 500 V, 750 V and 1000 V, respectively. The filmswere composed of agglomerates of the Eu₂O₃ nanocrystals, which wereapproximately 130-160 nm in diameter. These agglomerates formed from4×10¹⁵ NC cm⁻³ nanocrystal concentration were much larger than theobserved agglomerates deposited from the lower 2×10¹⁵ NC cm⁻³nanocrystal concentration (FIG. 3( b)). The agglomerate size wasconsistent across individual films and was nearly identical fordifferent applied voltages (FIGS. 7( a)-(d)). The smoothness of all thefilms was comparable. The RMS surface roughness, determined from ananalysis of the AFM images of the films, varied between about 1.6 and1.8 nm. This roughness was still smaller than the diameter of onenanocrystal. Thus, the films maintained a smooth topography as afunction of film thickness/applied voltage.

Additionally, EDS analysis of the films was performed to juxtapose theircompositions. EDS analyses were performed on small, cleaved sections ofthe EPD films placed onto silicon substrates rather than on the originalITO-coated glass substrates. This step was utilized since thecontribution of oxygen signal from the substrate dominated the oxygensignal from the Eu₂O₃ nanocrystal films. Silicon substrates were chosenbecause their EDS peaks do not coincide with the europium and oxygenpeaks. FIGS. 7( e)-(h) show the EDS graphs of the EPD films, depositedat 250 V, 500 V, 750 V and 1000 V, respectively. To compare thecomposition of the nanocrystals, it was monitored the intensity of theoxygen peak (K line: 0.52 keV) relative to the intensity of the europiumpeak (M line: 5.84 keV). The average ratio of intensities, 2.32±0.13,was within 5% of all four of the intensity ratios, which confirmed thatthe composition of the nanocrystals in the films did not change as afunction of, or because of, the applied voltage. Thus, these analysesconfirmed that the morphology, composition and topology of the film atthe microscopic level remained consistent as the film thicknessincreased.

In sum, in one aspect, transparent films of about 2.4 nm diameter Eu₂O₃nanocrystals were deposited successfully onto conducting substrates fromnanocrystal suspensions, prepared in hexane, using the EPD techniqueaccording to various embodiments of the present invention. The filmscomprised agglomerates (about 15 nm) of Eu₂O₃ nanocrystals, which likelyformed under the influence of the electric field applied during EPD. Thesmall size of the agglomerates scattered a small fraction of visiblelight, which reduced light scattering losses, and thus enhancedtransparency of the film (>80%) in the visible region. The films wereuniform, smooth and densely packed with a packing fraction of about 0.56(glassy packing regime). The films maintained very low RMS surfaceroughness (about 1.4 nm). The effect of the EPD process parameters(applied voltage and nanocrystal concentration) on growth of thesetransparent films was evaluated. The thickness of the nanocrystal filmsincreased with applied voltage and nanocrystal concentration; however,growth of transparent films of the Eu₂O₃ nanocrystals slowed down withdeposition time. The voltage drop across the film of insulating Eu₂O₃nanocrystals played a primary role and the depletion of nanocrystalsfrom the suspension played a secondary role in suppressing the growth ofthe films. Nonuniformity of the nanocrystal film increased with appliedvoltage and nanocrystal concentration, which was attributed to theeffect of a fringe electric field. Knowledge of the effect of EPDprocess parameters on the overall quality of the nanocrystal films mayprove helpful to the deposition of other nanocrystal systems. Insightgained into the growth of high quality electrophoretically depositedfilms of the colloidal nanocrystals would be beneficial in the designand production of nanocrystal-based optical, magnetic and electronicdevices.

Example 2

Gd₂O₃ Nanocrystal Films

Gadolinium oxide (Gd₂O₃) in its crystalline and amorphous phases hasbeen of research interest as a replacement gate oxide material forsilicon dioxide because of its high dielectric constant (κ=14) [58].Nanocrystals (NCs) of Gd₂O₃ have been investigated for theirapplications as a magnetic contrast agents [59, 60], and host materialsin Eu³⁺- and Tb³⁺-doped light emitting materials [61]. Recently,dielectric studies of amorphous Gd₂O₃ films, embedded with Gd₂O₃ NCs,revealed intriguing charge storage characteristics of the NCs [62].Similarly, metallic NCs (Au, Ru, Ni, and Co) [63-65] and semiconductingNCs (Si and Ge) [66, 67] have exhibited charge-storage characteristicswhen they were embedded in the gate oxide layer of ametal-oxide-semiconductor (MOS) structure for non-volatile memory (NVM)applications. NC confined states, the states at the interface ofNC-dielectric (i.e. NC surface), and the defect sites inside NCs areresponsible for charge-storage behavior [63, 68, 69]. Colloidal NCs maypossess similar charge-storage capabilities because of the unpassivatedsurface states that can arise due to the detachment of some fraction ofthe nanocrystals' surface capping ligands during cleaning procedure[70-72]. Dielectric studies of films composed entirely of colloidalGd₂O₃ NCs may provide insight into this subject. An effective technologyfor the fabrication of casts of particles, electrophoretic deposition(EPD) can produce densely-packed films of colloidal NCs at highdeposition rates [70, 73]. According to various embodiments of thepresent invention, EPD was employed to produce films consisting only ofcolloidal Gd₂O₃ NCs to be used as the gate oxide layer in MOSarchitecture. Capacitance-voltage (C-V) measurements of these MOSstructures exhibited hysteresis, illustrating the charge-storagecapabilities of the films. The dielectric constant of the NC films (κ)was determined from C-V measurements of the MOS structures withdifferent NC film thicknesses. Since the dielectric constant of the NCfilms depended on the packing density of NCs within the films, κsubsequently was used to determine the NC packing density.

Fabrication of MOS Capacitor Structures and Characterizations.

In one embodiment of the present invention, as shown in the schematic ofFIG. 8( a), MOS capacitor structures were fabricated, composed of filmsof ultra-small colloidal Gd₂O₃ NCs employed as the dielectric oxidelayer. The about 2.4 nm diameter Gd₂O₃ nanocrystals were synthesized viaa two-stage solution-phase technique and were capped with oleic acid[71]. The as-synthesized NCs were cleaned in ethanol using aprecipitation-centrifugation sequence, described elsewhere [71]. Afterpurifying the Gd₂O₃ NCs, they were suspended in hexane and then weredeposited onto p-(100) silicon substrates [epi layer: 20-40 Ω·cm andsubstrate: 0.005-0.025 Ω·cm] via electrophoretic deposition. For EPD, apair of silicon electrodes (25 mm×13 mm) was mounted in a parallel-plateconfiguration with a gap of about 5 mm. A DC voltage of 500 V wasapplied to the electrode pair using a Keithley 6517A electrometer. Thevoltage was applied for 15 min during the deposition and, subsequently,for 5 min during an air drying step of the film to improve densificationof the film [70, 74]. The NCs deposited on both the cathode and theanode. FIG. 8( b) shows an image of the NC film (anode) captured using aNanoscope III atomic force microscope (AFM). The film was continuous andtopologically smooth with root mean square (RMS) roughness of about 1.6nm, which was smaller than the diameter of the nanocrystals (about 2.4nm). An image of the nanocrystals acquired with Philips CM 20transmission electron microscope (TEM) is shown as an inset to FIG. 8(b). It was confirmed the deposition of the Gd₂O₃ NCs by performingenergy dispersive spectroscopy (EDS) of the film using a Link ISISseries 300 microanalysis system. Shown in FIG. 8( c), the EDS spectrumof the film exhibits the gadolinium, oxygen, carbon, and silicon peaks,which are present due to the oleic acid-capped Gd₂O₃ NCs on the siliconsubstrate. To complete the fabrication of the MOS capacitors, aluminumcontacts (500 μm diameter and 300 nm thick) were deposited on the Gd₂O₃NC film via e-beam evaporation of aluminum using a shadow mask. Aluminumwas chosen as the gate material because of its suitable work functionand cost effectiveness. FIG. 8( d) shows the image, which is a top view,of the MOS capacitors recorded using a Hitachi S-4200 scanning electronmicroscope (SEM). MOS capacitors were fabricated with different oxidelayer (NC film) thicknesses (116±10 nm, 179±10 nm, 276±10 nm, and 397±15nm), which were verified using a Veeco Dektak 150 surface profiler. Tovary thickness of the NC film, EPD suspensions of different NCconcentrations (1.0×10¹⁵ NC/cm³, 1.5×10¹⁵ NC/cm³, 2.0×10¹⁵ NC/cm³, and2.5×10¹⁵ NC/cm³) were employed.

C-V Measurements, and Calculations of the Dielectric Constant (κ) andthe Packing Density.

High-frequency C-V measurements of the capacitors were performed at afrequency of 1 MHz and at a sweep rate of 50 mV/s, using a Keithley 590CV analyzer on a Signatone probe station. The gate voltage was sweptfrom −10 V (accumulation) to +5 V (inversion) and back to −10 V(accumulation). The capacitors were biased at −10 V for 15 min prior tothe forward sweep and were biased at +5 V for 1 min prior to the reversesweep. FIG. 9 shows the C-V characteristics of capacitors fabricatedfrom 116 nm thick NC films, deposited on the anode and cathode. C-Vcharacteristics of both capacitors are similar to that of a typical MOScapacitor with distinct accumulation, depletion, and inversion regions.The MOS capacitors exhibited a clockwise hysteresis in their C-Vcharacteristics as they were biased through theaccumulation-inversion-accumulation regions. The observed hysteresisindicated the presence of charge carriers within the NC film. The chargecarriers could be immobile charges, arising from the unpassivatedsurface sites (Gd³⁺ and O²⁻) of the NCs, or mobile charges (electrons orholes), injected into the NC film. The presence of positive chargesshifts the flat-band voltage (V_(FB)) in negative direction, while thepresence of negative charges shifts it in positive direction. Assumingno charges in the NC film, the ideal flat-band voltage (V_(FB) ^(Ideal))of −0.88 V was based on the work functions of Al and Si and the dopingconcentration in the epi layer. A larger negative shift in V_(FB)(ΔV_(FB) is about −3.92 V, anode) during reverse sweep(inversion-accumulation) than the positive shift in V_(FB) (ΔV_(FB) isabout 0.05 V, anode) during forward sweep (accumulation-inversion)suggested the presence of more positive charges in the NC film.Electrons were injected into the NC film from the gate electrode in theaccumulation region, while electrons were subsequently extracted(equivalent to injection of holes) from the NC film into the gateelectrode in the inversion region. To compare the charge-storage in theNC films deposited on the anode and cathode, it was measured the widthof the hysteresis window (ΔV) for the two NC films [3.97 V (anode) and4.19 V (cathode)]. These values were within the statistical uncertainty(±0.13 V) when multiple MOS capacitors were characterized. Similar toother metal [64], semiconductor [66, 67], and insulator [62] NC-embeddedMOS capacitor structures, charge-storage was observed in the Gd₂O₃ NCfilms formed according to various embodiments of the present invention.

FIG. 10 shows C-V characteristics of the MOS capacitors for NC films ofdifferent thicknesses. The oxide capacitance, C_(OX), in theaccumulation region decreased with increased NC film thickness, as wasexpected. For a given gate dielectric material (typically oxide), theoxide capacitance (in accumulation) has a linear relationship with theinverse of the gate oxide thickness, as stated in equation 1 set forthbelow:

$\begin{matrix}{C_{OX} = \frac{A \times ɛ_{OX}}{t_{OX}}} & (1)\end{matrix}$In this expression, C_(OX) is the oxide capacitance (F), A is the gatearea (cm²), t_(OX) is the oxide thickness (cm), and ∈_(OX) is thepermittivity of the oxide material (F/cm). From the thickness andcapacitance of the Gd₂O₃ NC film, the permittivity of the NC film can bedetermined. FIG. 11 shows a graph of the NC film capacitance as afunction of the inverse of the NC film thickness for MOS capacitors withdifferent NC film thicknesses. The data exhibited good agreement withthe linear trend. The dielectric permittivity of the NC film wasextracted from slope of the linear fit, given the area of the gate. Thedielectric constant, κ, of the Gd₂O₃ NC film was calculated using therelation, κ=∈_(OX)/∈₀ and was found to be 3.90±0.06. Since the NC filmcomprised Gd₂O₃ (κ=14.0), oleic acid (κ=2.5), and air (κ=1.0), theeffective dielectric constant of the NC film depended on the volumetricfractions of each component in the film. Good agreement between the dataand the liner fit suggested that all of the NC films, cast fromsolutions with different NC concentrations, possessed comparable NCpacking fractions. It was calculated the volumetric packing fractions ofthe NC film using a three-component Bruggeman model for the dielectricconstant (Equations 2 and 3):

$\begin{matrix}{{{f_{air}\frac{\kappa_{air} - \kappa_{{NC}\mspace{14mu}{film}}}{\kappa_{air} + {2\kappa_{{NC}\mspace{14mu}{film}}}}} + {f_{oleic}\frac{\kappa_{oleic} - \kappa_{{NC}\mspace{14mu}{film}}}{\kappa_{oleic} + {2\kappa_{{NC}\mspace{14mu}{film}}}}} + {f_{{Gd}_{2}O_{3}}\frac{\kappa_{{Gd}_{2}O_{3}} - \kappa_{{NC}\mspace{14mu}{film}}}{\kappa_{{Gd}_{2}O_{3}} + {2\kappa_{{NC}\mspace{14mu}{film}}}}}} = 0} & (2) \\{{f_{air} + f_{oleic} + f_{{Gd}_{2}O_{3}}} = 1} & (3)\end{matrix}$In the expression, volume fractions of air, NC film, oleic acid, andGd₂O₃ are given as f_(air), f_(NC film), f_(oleic), and f_(Gd) ₂ _(O) ₃, respectively. Dielectric constants of air, NC film, and Gd₂O₃ aregiven as κ_(air), κ_(NC film), κ_(oleic), and κ_(Gd) ₂ _(O) ₃ ,respectively. Based on the coverage of oleic acid on the surface of aspherical NC core, a relationship was formed between the volumetricfractions of the oleic acid surfactant and the Gd₂O₃ NC core as statedin equation 4, where R₁ is radius of the Gd₂O₃ NC core (R₁=1.2 nm±0.1),and R₂ is the radius of NC core plus thickness of the oleic acid layer(0.3±0.1 nm):f _(oleic) =f _(Gd) ₂ _(O) ₃ [(R ₂ /R ₁)³−1]  (4)Volumetric fractions, f_(Gd) ₂ _(O) ₃ =0.34±0.02, f_(air)=0.34±0.08, andf_(oleic)=0.32±0.10, were calculated from equations 2-4. The summedpacking fraction for the nanocrystals (Gd₂O₃ NC core plus oleic acid) is0.66±0.08 and resides within the glassy packing regime for closelypacked spheres [75]. Thus, EPD can produce densely packed, glassy filmsof ultra-small nanocrystals that exhibit potential charge storagecapabilities, surmised from their packing density and their effectivedielectric constant.Dielectric Properties of the Colloidal Gd₂O₃ NC Films.

Accordingly, in another aspect, MOS capacitor structures with colloidalGd₂O₃ NC film as oxide layer were fabricated according to variousembodiments of the present invention. Uniformly deposited films of thepurified Gd₂O₃ NCs were produced via electrophoretic deposition. C-Vmeasurements of the MOS capacitors exhibited clockwise hysteresis thatsuggested charge-storage within the NC films. The NC films, deposited onthe anode and cathode, had similar charge-storage properties. NC filmswith different thicknesses showed charge-storage behavior. Dielectricconstant (κ=3.90) of the NC films was calculated from the C-Vmeasurements of the MOS capacitors. Packing density of the NCs withinthe film (0.66±0.08) was calculated from the dielectric constant of theNC film and was found to be within glassy-packing regime, as expectedfor the films deposited via EPD.

The foregoing description of the exemplary embodiments of the inventionhas been presented only for the purposes of illustration and descriptionand is not intended to be exhaustive or to limit the invention to theprecise forms disclosed. Many modifications and variations are possiblein light of the above teaching.

The embodiments were chosen and described in order to explain theprinciples of the invention and their practical application so as toenable others skilled in the art to utilize the invention and variousembodiments and with various modifications as are suited to theparticular use contemplated. Alternative embodiments will becomeapparent to those skilled in the art to which the present inventionpertains without departing from its spirit and scope. Accordingly, thescope of the present invention is defined by the appended claims ratherthan the foregoing description and the exemplary embodiments describedtherein.

LIST OF REFERENCES

-   [1] Dabbousi B 0, Bawendi M G, Onitsuka O and Rubner M F, 1995 Appl.    Phys. Lett. 66 1316-8.-   [2] Shevchenko E V, Talapin D V, Kotov N A, O'Brien S and Murray C B    2006Nature 439 55-9.-   [3] Mirkin C A, Letsinger R L, Mucic R C and Storhoff J J 1996,    Nature 382 607-9.-   [4] Murray C B, Kaganii C R and Bawendi M G 1995 Science, 270    1335-8.-   [5] Dabbousi B 0, Murray C B, Rubner M F and Bawendi M G, 1994 Chem.    Mater. 6 216-9.-   [6] Kim F, Kwan S, Akana J and Yang P 2001J. Am. Chem. Soc., 123    4360-1.-   [7] Islam M A and Herman I P 2002 Appl. Phys. Lett. 80 3823-5.-   [8] Singh I, Kaya C, Shaffer M S P, Thomas B C and Boccaccini A R    2006 J. Mater. Sci. 41 8144-51.-   [9] Mahajan S V, Hasan S A, Cho J, Shaffer M S P, Boccaccini A R and    Dickerson J H 2008 Nanotechnology 19 195301.-   [10] Maenosono S, Okubo T and Yamaguchi Y 2003 J. Nanopart. Res. 5    5-15.-   [11] Giersig M and Mulvaney P 1993 J. Phys. Chem. 97 6334-6.-   [12] Teranishi T, Hosoe M, Tanaka T and Miyake M 1999 J. Phys. Chem.    B 103 3818-27.-   [13] Wong E M and Searson P C 1999 Appl. Phys. Lett. 74 2939-41.-   [14] Dor S, Ruhle S, Ofir A, Adler M, Grinis L and Zaban A 2009,    Colloids Surf A 342 70-5.-   [15] Ferrari B, Bartret A and Baudin C 2009 J. Eur. Ceram. Soc., 29    1083-92.-   [16] Mahajan S V, Cho J, Shaffer M S P, Boccaccini A R and Dickerson    J H 2010 J. Eur. Ceram. Soc. 30 1145-50.-   [17] Castro Y, Ferrari B, Moreno R and Duran A 2004 Surf Coat.    Technol. 182 199-203.-   [18] Jung D, Tabellion J and Clasen R 2006 Key Eng. Mater., 314    81-8.-   [19] Islam M A, Xia Y Q, Steigerwald M L, Yin M, Liu Z, O'Brien S,    Levicky R and Herman I P 2003 Nano Lett., 3 1603-6.-   [20] Somarajan S, Hasan S A, Adkins C T, Harth E and Dickerson J H    2008 J. Phys. Chem. B 112 23-8.-   [21] Zhao G F, Ishizaka T, Kasai H, Hasegawa M, Furukawa T,    Nakanishi H and Oikawa H 2009 Chem. Mater. 21 419-24.-   [22] Boccaccini A R, Cho J, Roether J A, Thomas B J C, Minay E J and    Shaffer M S P 2006 Carbon 44 3149-60.-   [23] Cho J, Schaab S, Roether J A and Boccaccini A R 2008 J.    Nanopart. Res. 10 99-105.-   [24] Du C S and Pan N 2006 Nanotechnology 17 5314-8.-   [25] Du C S and Pan N 2006 J. Power Sources 160 1487-94.-   [26] Thomas B J C, Boccaccini A R and Shaffer M S P 2005J. Am.    Ceram. Soc. 88 980-2.-   [27] Thomas B J C, Shaffer M S P, Freeman S, Koopman M, Chawla K K    and Boccaccini A R 2006 Key Eng. Mater., 314 141-6.-   [28] Islam M A and Xia S G 2009 J. Phys.: Condens. Matter 21 8.-   [29] Murray C B, Kagan C R and Bawendi M G 2000 Annu. Rev. Mater.    Sci. 30 545-610.-   [30] Park J, Joo J, Kwon S G, Jang Y and Hyeon T 2007 Angew. Chem.    Int. Edn 46 4630-60.-   [31] Mahajan S V and Dickerson J H 2007 Nanotechnology, 18 325605.-   [32] Raue R, Vink A T and Welker T 1989 Philips Tech. Rev. 44,    335-47.-   [33] Mochizuki S, Nakanishi T, Suzuki Y and Ishi K 2001 Appl. Phys.    Lett. 79 3785-7.-   [34] Hamaker H C 1940 Trans. Faraday Soc. 36 279-83.-   [35] Besra L and Liu M 2007 Prog. Mater. Sci. 52 1-61.-   [36] De D and Nicholson P S 1999 J. Am. Ceram. Soc. 82 3031-6.-   [37] Ferrari B, Sanchez-Herencia A J and Moreno R 1998 Mater. Lett.    35 370-4.-   [38] Sarkar P, Huang X N and Nicholson P S1992 J. Am. Ceram. Soc. 75    2907-9.-   [39] Sarkar P, Huang X N and Nicholson P S1993 J. Am. Ceram. Soc. 76    1055-6.-   [40] Tang F Q, Uchikoshi T, Ozawa K and Sakka Y 2002 Mater. Res.    Bull. 37 653-60.-   [41] Islam M A, Xia Y Q, Telesca D A, Steigerwald M L and Herman I P    2004 Chem. Mater. 16 49-54.-   [42] Cushing B L, Kolesnichenko V L and O'Connor C J 2004, Chem.    Rev. 104 3893-946.-   [43] Sigmund W M, Bell N S and Bergstrom L 2000J. Am. Ceram. Soc. 83    1557-74.-   [44] Vandeperre L, Zhao C and Van der Biest 0 1999 6th Conf. and    Exhibition of the European Ceramic Society (Brighton: Institute of    Materials) pp 69-74.-   [45] Fowkes F M and Pugh R J 1984 ACS Symp. Ser. vol 240    (Washington, D.C.: American Chemical Society) pp 331-54.-   [46] Morrison I D 1993 Colloids Surf A 71 1-37.-   [47] Morrison I D and Ross S 2002 Colloidal Dispersions: Suspension,    Emulsions, and Foams (New York: Wiley-Interscience).-   [48] Shim M and Guyot-Sionnest P 1999 J. Chem. Phys., 111 6955-64.-   [49] Basu R N, Randall C A and Mayo M J 2001 J. Am. Ceram. Soc., 84    33-40.-   [50] Chen F and Liu M 2001 J. Eur. Ceram. Soc. 21 127-34.-   [51] Carnall W T, Fields P R and Rajnak K 1968 J. Chem. Phys., 49    4450-5.-   [52] Dieke G H 1968 Spectra and Energy Levels of Rare Earth Ions in    Crystals (New York: Wiley).-   [53] Wakefield G, Keron H A, Dobson P J and Hutchison J L 1999, J.    Colloid Interface Sci. 215 179-82.-   [54] Mahajan S V, Kavich D W, Redigolo M L and Dickerson J H 2006 J.    Mater. Sci. 41 8160-5.-   [55] Scott G D and Kilgour D M 1969 J. Phys. D: Appl. Phys. 2 863-6.-   [56] Wang Y C, Leu I C and Hon M H 2004 J. Am. Ceram. Soc. 87 84-8.-   [57] Zhitomirsky I and Galor L 1997 J. Mater. Sci., Mater. Med., 8    213-9.-   [58] J. Kwo, M. Hong, A. R. Kortan, K. T. Queeney, Y. J.    Chabal, J. P. Mannaerts, T. Boone, J. J. Krajewski, A. M. Sergent,    and J. M. Rosamilia, Appl. Phys. Lett. 77, 130 (2000).-   [59] J. L. Bridot, A. C. Faure, S. Laurent, C. Riviere, C.    Billotey, B. Hiba, M. Janier, V. Josserand, J. L. Coll, L. Vander    Elst, R. Muller, S. Roux, P. Perriat, and O. Tillement, J. Am. Chem.    Soc. 129, 5076 (2007).-   [60] J. Y. Park, M. J. Baek, E. S. Choi, S. Woo, J. H. Kim, T. J.    Kim, J. C. Jung, K. S. Chae, Y. Chang, and G. H. Lee, ACS Nano 3,    3663 (2009).-   [61] R. Bazzi, M. A. Flores-Gonzalez, C. Louis, K. Lebbou, C.    Dujardin, A. Brenier, W. Zhang, O. Tillement, E. Bernstein, and P.    Perriat, J. Lumin. 102, 445 (2003).-   [62] J. C. Wang, C. S. Lai, Y. K. Chen, C. T. Lin, C. P.    Liu, M. R. S. Huang, and Y. C. Fang, Electrochem. Solid State Lett.    12, H202 (2009).-   [63] W. H. Guan, S. B. Long, M. Liu, Z. G. Li, Y. Hu, and Q. Liu, J.    Phys. D-Appl. Phys. 40, 2754 (2007).-   [64] B. Park, K. Cho, H. Kim, and S. Kim, Semicond. Sci. Technol.    21, 975 (2006).-   [65] S.-S. Yim, M.-S. Lee, K.-S. Kim, and K.-B. Kim, Appl. Phys.    Lett. 89, 093115 (2006).-   [66] A. Kanjilal, J. L. Hansen, P. Gaiduk, A. N. Larsen, N.    Cherkashin, A. Clayerie, P. Normand, E. Kapelanakis, D. Skarlatos,    and D. Tsoukalas, Appl. Phys. Lett. 82, 1212 (2003).-   [67] S. Duguay, J. J. Grob, A. Slaoui, Y. Le Gall, and M.    Amann-Liess, J. Appl. Phys. 97, 5 (2005).-   [68] Y. Liu, T. P. Chen, L. Ding, S. Zhang, Y. Q. Fu, and S.    Fung, J. Appl. Phys. 100, 3 (2006).-   [69] Y. Shi, K. Saito, H. Ishikuro, and T. Hiramoto, J. Appl. Phys.    84, 2358 (1998).-   [70] M. A. Islam and I. P. Herman, Appl. Phys. Lett. 80, 3823    (2002).-   [71] S. V. Mahajan and J. H. Dickerson, Nanotechnology 18, 325605    (2007).-   [72] S. A. Hasan, D. W. Kavich, and J. H. Dickerson, Chem. Commun.    3723 (2009).-   [73] S. V. Mahajan, S. A. Hasan, J. Cho, M. S. P. Shaffer, A. R.    Boccaccini, and J. H. Dickerson, Nanotechnology 19, 8 (2008).-   [74] S. A. Hasan, D. W. Kavich, S. V. Mahajan, and J. H. Dickerson,    Thin Solid Films 517, 2665 (2009).-   [75] G. D. Scott and D. M. Kilgour, J. Phys. D-Appl. Phys. 2, 863    (1969).-   [76] Shionoya S, and Yen W. M., 1999, Phosphor Handbook (Boca Raton:    CRC Press).-   [77] Kwo J., Hong M., Kortan A. R., Queeney K. L., Chabal Y. J.,    Opila R. L., Muller D. A., Chu S. N. G., Sapjeta B. J., Lay T. S.,    Mannaerts J. P., Boone T., Krautter H. W., Krajewski J. J.,    Sergnt A. M., and Rosamilia J. M., 2001, Properties of high kappa    gate dielectrics Gd ₂ O ₃ and Y ₂ O ₃ for Si, Journal of Applied    Physics 89 3920-7.-   [78] Blasse G. and Grabmaier B. C., 1994, Luminescent Materials    (Berlin: Springer).

What is claimed is:
 1. A metal-oxide-semiconductor (MOS) capacitor,comprising: (a) a silicon substrate having a first surface; (b) a filmof lanthanide oxide nanoparticles functioning as a charge storage of theMOS capacitor and formed by electrophoretically depositing pre-formedlanthanide oxide nanoparticles directly on the first surface of thesilicon substrate; and (c) an aluminum film formed on the film oflanthanide oxide nanoparticles.
 2. The metal-oxide-semiconductor (MOS)capacitor of claim 1, wherein the silicon substrate comprises p-typesilicon.
 3. The metal-oxide-semiconductor (MOS) capacitor of claim 2,wherein the first surface of the silicon substrate is a p-(100) surfaceof silicon.
 4. The metal-oxide-semiconductor (MOS) capacitor of claim 1,wherein the lanthanide oxide nanoparticles comprise europium oxide(Eu₂O₃) nanoparticles or gadolinium oxide (Gd₂O₃) nanoparticles.
 5. Themetal-oxide-semiconductor (MOS) capacitor of claim 1, wherein the filmof lanthanide oxide nanoparticles has a thickness ranging from about 50to about 500 nm.
 6. The metal-oxide-semiconductor (MOS) capacitor ofclaim 1, wherein the aluminum film has a thickness of about 300 nm. 7.The metal-oxide-semiconductor (MOS) capacitor of claim 1, wherein thealuminum film is formed on the film of lanthanide oxide nanoparticlesusing electron-beam evaporation.
 8. The metal-oxide-semiconductor (MOS)capacitor of claim 1, wherein the film of lanthanide oxide nanoparticlescomprises randomly close-packed lanthanide oxide nanoparticles with apacking density of about 66%.